This invention relates to a process for producing high-purity Mn materials and also to high-purity Mn materials for thin film deposition and its production process. More particularly, this invention relates to high-purity Mn materials which can be used as starting materials for Mn alloy materials for antiferromagnetic thin film deposition.
Magnetic recording devices such as hard disks for computers have in recent years been rapidly reduced in size and boosted in capacity. Indications are that their recording density will be as high as 20 Gb/in2 in a few years. Keeping pace with the trend, reproducing heads of magnetoresistance effect (AMR) type are coming into use, supplanting the conventional induction type heads that are close to their limits of utility. The AMR heads are expected to attain rapid growth worldwide with the expansion of demand as in the personal computer market. There is a strong possibility that giant magnetoresistance effect (GMR) type heads that promise even greater densities in recording will come into practical use in years to come.
The GMR heads use spin valve membranes, and Mn alloys are drawing attention as materials for antimagnetic membranes to serve as the membranes.
As antimagnetic membranes for spin valve membranes, Mn alloys, particularly Mn-noble metal alloys, are under investigation. They are usually formed by sintering or melting. However, when commercially available electrolytic Mn is employed as the starting material for sputtering targets, melting causes bumping and spattering of molten Mn. In addition, much slags are formed, the cast ingot has a large cavity, and the yield as a target material is poor.
Sintering presents problems of abundant gas release and rather low sinter density.
Conventional alloys based on Mn too have problems of gas evolution on sputtering, production of objectionable particles, and inadequate corrosion resistance.
This invention is aimed at providing means for obtaining a high-purity Mn material for targets which attain high yield and is best suited for antiferromagnetic thin film deposition so as to obtain a high-purity Mn material which contains a total of not more than 100 ppm impurity metallic elements, not more than 200 ppm oxygen, not more than 50 ppm nitrogen, not more than 50 ppm S, and not more than 100 ppm C.
After intensive research on possible solutions to the foregoing problems, the present inventors have just found that impurity metallic elements in Mn have important bearings upon the molten state of the metal and that combining premelting with vacuum distillation makes it possible to decrease those impure contents substantially. It has now been found that the high-purity Mn materials thus obtained are limited in particle generation during sputtering and are excellently corrosion-resistant.
On the basis of these findings, this invention provides:
1. A process for producing a high-purity Mn material comprising the steps of premelting crude Mn at 1250-1500xc2x0 C. and then vacuum distilling the melt at 1100-1500xc2x0 C.
2. A process according to 1 above, wherein the degree of vacuum during the vacuum distillation ranges from 5xc3x9710xe2x88x926 torr to 10 torrs.
3. A process according to 1 or 2 above, wherein a crucible for use in the vacuum distillation is a double crucible, which consists of an inner crucible, an outer crucible, and a carbon felt packed in the space therebetween.
4. A high-purity Mn material for thin film deposition, characterized in that it contains a total of not more than 100 ppm impurity metallic elements, not more than 200 ppm oxygen, not more than 50 ppm nitrogen, not more than 50 ppm S, and not more than 100 ppm C.
5 . A high-purity Mn material for thin film deposition, characterized in that it contains a total of not more than 50 ppm impurity metallic elements, not more than 100 ppm oxygen, not more than 10 ppm nitrogen, not more than 10 ppm S, and not more than 50 ppm C.
The crude Mn as a starting material for the high-purity Mn material according to this invention may be a commercially available electrolytic Mn.
The crude Mn is premelted at 1250-1500xc2x0 C. The premelting is carried out using a crucible of MgO, Al2O3 or the like, in an inert gas atmosphere for a holding time of at least one hour. A temperature below 1250xc2x0 C. is undesirable because it does not melt Mn, as is a temperature above 1500xc2x0 C. which intensifies contamination from the crucible and evaporation of Mn. A holding time of less than one hour is undesirable because it leaves part of Mn unmelted.
The premelting here is intended to remove volatile ingredients from the material.
Following the premelting, vacuum distillation is performed at 1100-1500xc2x0 C. At below 1100xc2x0 C. the distillation time is prolonged to excess and at above 1500xc2x0 C. the evaporation rate is so high that the melt tends to catch up impurities.
The degree of vacuum for vacuum distillation ranges from 5xc3x9710xe2x88x926 to 10 torrs. If it is less than 5xc3x9710xe2x88x926 torr, no condensate will result, but if it exceeds 10 torrs Mn distillation takes too much time.
The distillation time suitably ranges from 10 to 20 minutes.
The crucible for vacuum distillation is desirably a double crucible of Al2O3 or the like. Particularly desirable is one consisting of an inner crucible and an outer crucible, with carbon felt packed in the space between the two crucibles. In the absence of carbon felt, much deposit will cover the inner wall surface of the inner crucible of Al2O3 or the like, reducing the yield of the distillate accordingly. The carbon felt packed between the inner and outer crucibles substantially decreases the deposit on the inner wall of the inner Al2O3 crucible, with a consequent increase in the yield of the distillate.
The vacuum distillation is desirably carried on until the residual amount is less than about 50% of the original amount of the charge.
The high-purity Mn material thus obtained has remarkably decreased impurity contents and is best suited as an Mn material for magnetic thin film deposition. It contains a total of not more than 100 ppm impurity metallic elements, not more than 200 ppm oxygen, not more than 50 ppm nitrogen, not more than 50 ppm S, and not more than 100 ppm C. Impurity metallic element contents are desired to be the least possible, because they deteriorate the magnetic properties and can decrease the corrosion resistance of the product. The total amount should be not more than 100 ppm, preferably not more than 50 ppm. Of other impurities, oxygen and sulfur are particularly responsible for a decrease in corrosion resistance, and therefore the oxygen content should be reduced to not more than 200 ppm, preferably not more than 100 ppm, and the sulfur content be reduced to not more than 50 ppm, preferably not more than 10 ppm.
Nitrogen and carbon are deemed not merely responsible for reduced corrosion resistance but as factors contributing to the generation of unwanted particles during sputtering. For these reasons the nitrogen content should be kept below 50 ppm, preferably below 10 ppm, and the carbon content be kept below 100 ppm, preferably below 50 ppm.
The high-purity Mn material obtained under this invention may be alloyed with another metal, e.g., Fe, Ir, Pt, Pd, Rh, Ru, Ni, Cr, or Co, to provide a material, such as a sputtering target, for magnetic thin film deposition. In such a case, needless to say, the alloying element to be combined with Mn too should be of the highest purity possible; when a commercially available product is employed it is desirably of a purity as high as 99.99% or above. When necessary, the alloying element should be freed from gaseous and volatile ingredients by vacuum degassing or other similar treatment.
The high-purity Mn material obtained as above and an alloying element other than Mn are melted together for alloying and then cast into ingot. The high-purity Mn material of this invention reduces the frequency of bumping and produces an ingot of smaller cavity than usual.
The alloy ingot thus obtained can be machined to form a sputtering target material.
The sputtering target is then used in sputtering to deposit a magnetic thin film over a substrate.